4:30 PM - 4:45 PM
[SGC32-10] Development of an On-site Analysis Method for Helium Isotopes Using a Portable Mass Spectrometer
Keywords:helium isotope ratio, volcano monitoring, on-site measurement, MULTUM
The helium isotope ratio (3He/4He) differs among geochemical reservoirs in Earth’s interior, such as the crust, atmosphere, and mantle. Helium in volcanic gases is a mixture derived from these three components. When volcanic activity increases, the contribution of mantle-derived components increases, and the 3He/4He ratio of volcanic gases becomes higher. Regarding the monitoring of volcanic activity, there are some cases where 3He/4He changes have been observed earlier than in other precursory geophysical observations. One of them is reported by Padrón et al. (2013), who observed increases in diffusive 4He emission and 3He/4He ratio a few weeks before an increase in seismic energy release followed by the eruption at El Hierro, Canary Island.
The conventional method of measuring noble gases, including helium, use a laboratory-installed magnetic sector-type mass spectrometer. The process of noble gas measurement is complicated and time-consuming, constraining the daily sample throughput. Improving temporal resolution is a challenge for using noble gas analysis of volcanic gases as a volcano monitoring. The purpose of this study is to simplify and enhance the efficiency of the measurement process of 3He/4He ratios of volcanic gases using a portable mass spectrometer, to establish an on-site measurement, and to implement it practically as a volcano monitoring technique.
The portable instrument that makes this possible is a multi-turn time-of-flight mass spectrometer (MULTUM), which is compact and has high mass resolution. Previous studies have shown that 3He/4He ratio can be measured with 10% accuracy using a helium standard gas and some volcanic gases compared to that obtained with a magnetic sector-type mass spectrometer (Hattori et al., 2022). The superior resolution of MULTUM eliminates the need to separate each noble gas element before introducing it into the mass spectrometer. For example, the system’s resolution enables the measurement of the 4He/20Ne ratio, often used as an indicator of atmospheric mixing, by distinguishing 20Ne+ from 40Ar++ without separating argon from neon and helium.
The process of noble gas measurement needs to remove active gases and separate noble gases by element. Active gases, such as water vapor, carbon dioxide, and volatile hydrocarbon compounds, are removed by titanium and zirconium metal pieces (Ti-Zr getter) heated from 700 to 800℃, while hydrogen is absorbed by the Ti-Zi getter cooled to room temperature (Sumino, 2015). Argon, being more abundant than other noble gases, krypton and xenon are adsorbed onto an activated charcoal trap cooled at liquid nitrogen temperature. In this study, we only used a Zr-Al alloy getter pump (NP-10, SAES getters), which can be heated to remove active gases, and we did not use charcoal to simplify the sample purification line.
We report the results of measurements on board the ship during the YK23-16S cruise of YOKOSUKA, JAMSTEC, at the Okinawa Trough, alongside those obtained using a magnetic field mass spectrometer (conventional method) in the laboratory for comparison. The samples were seafloor hydrothermal water collected by the Shinkai 6500 submersible. Of the eight samples compared, only four showed results consistent with each other by the two methods within the analytical errors, and the errors were large for samples with high 40Ar/4He ratios. The simplified line resulted in a longer purification time for larger gas volume samples, and the counting rate of helium ions was lower than expected. The decline in helium counting rates was considered to be due to the lower ionization efficiency of helium compared to argon. Because this would also affect 3He/4He ratio analysis, the influence of 40Ar on the measured 3He/4He ratio was examined. The 3He/4He ratio declined as 40Ar increased. In conclusion, even in measurements using portable equipment, it is necessary to have a function to remove argon from the sample gas to be analyzed for 3He/4He ratio.
The conventional method of measuring noble gases, including helium, use a laboratory-installed magnetic sector-type mass spectrometer. The process of noble gas measurement is complicated and time-consuming, constraining the daily sample throughput. Improving temporal resolution is a challenge for using noble gas analysis of volcanic gases as a volcano monitoring. The purpose of this study is to simplify and enhance the efficiency of the measurement process of 3He/4He ratios of volcanic gases using a portable mass spectrometer, to establish an on-site measurement, and to implement it practically as a volcano monitoring technique.
The portable instrument that makes this possible is a multi-turn time-of-flight mass spectrometer (MULTUM), which is compact and has high mass resolution. Previous studies have shown that 3He/4He ratio can be measured with 10% accuracy using a helium standard gas and some volcanic gases compared to that obtained with a magnetic sector-type mass spectrometer (Hattori et al., 2022). The superior resolution of MULTUM eliminates the need to separate each noble gas element before introducing it into the mass spectrometer. For example, the system’s resolution enables the measurement of the 4He/20Ne ratio, often used as an indicator of atmospheric mixing, by distinguishing 20Ne+ from 40Ar++ without separating argon from neon and helium.
The process of noble gas measurement needs to remove active gases and separate noble gases by element. Active gases, such as water vapor, carbon dioxide, and volatile hydrocarbon compounds, are removed by titanium and zirconium metal pieces (Ti-Zr getter) heated from 700 to 800℃, while hydrogen is absorbed by the Ti-Zi getter cooled to room temperature (Sumino, 2015). Argon, being more abundant than other noble gases, krypton and xenon are adsorbed onto an activated charcoal trap cooled at liquid nitrogen temperature. In this study, we only used a Zr-Al alloy getter pump (NP-10, SAES getters), which can be heated to remove active gases, and we did not use charcoal to simplify the sample purification line.
We report the results of measurements on board the ship during the YK23-16S cruise of YOKOSUKA, JAMSTEC, at the Okinawa Trough, alongside those obtained using a magnetic field mass spectrometer (conventional method) in the laboratory for comparison. The samples were seafloor hydrothermal water collected by the Shinkai 6500 submersible. Of the eight samples compared, only four showed results consistent with each other by the two methods within the analytical errors, and the errors were large for samples with high 40Ar/4He ratios. The simplified line resulted in a longer purification time for larger gas volume samples, and the counting rate of helium ions was lower than expected. The decline in helium counting rates was considered to be due to the lower ionization efficiency of helium compared to argon. Because this would also affect 3He/4He ratio analysis, the influence of 40Ar on the measured 3He/4He ratio was examined. The 3He/4He ratio declined as 40Ar increased. In conclusion, even in measurements using portable equipment, it is necessary to have a function to remove argon from the sample gas to be analyzed for 3He/4He ratio.